Mmmnn, I can think of a couple of ways - they gave the number to an informant, for example. Or they called someone while working a case and the number came up on caller ID. I know that many "official" cell phones come up "private" - but that in itself is a giveaway. Why not just keep a bank of cell phones for plainclothes to use? I agree with another poster, unless you are truly undercover - and few LMPD officers really work undercover - they work plainclothes - there's a difference - true undercover